Hello, I'm an avid league player and have been playing since Season 2. Fiora is my main and most played champion with 368k mastery points. She is a great champion but really hard mechaniclly, but if you invest time in her you will get the grip of the champion. I will list some pros and cons of the champion.

Pros:

High sustain
Great splitpusher
W is a miracle maker
Great late game
Fun champion to play and you will learn something new every game

Cons:

Squishy
Depends on W
It's hard to come back if you fall behind
Hard to become a good Fiora

[1st tree]
Conqueror is by far the best rune for Fiora from the precision tree.

Triumph is really good in lane because it helps you turning 2v1s, surviving when the fight is close, and good for split pushing late game, since it helps you to keep pushing if you were fighting previously. It is also good for 2v1s late game. Overheal isn't so good, because the shield is weak early and for late game you already have Bloodthrister and the shield falls off very fast and doesn't have that big of a use.

I take Alacrity because it makes her feel smoother and faster in her combos, but if you plan to go Trinity Force you can switch it for Bloodline, but I think bloodline is overkill since you almost always get a 2nd life steal item and she has build in lifesteal, so a better option is Tenacity. Tenacity is also the best option against teams that have too much CC and where riposte wouldn't be enough.

I take Last Stand since Coup De Grace has been overnerfed and doesn't help that much especially on Fiora, Cut Down is way too hard to make use of and is only good on ADCs probably. Last Stand is also really good for turning 2v1s and it's really easy to make use of the first part of the rune.


[2nd tree]
For the 2nd tree is usually take Bone Plating and Chrysalis. Bone Plating is really good at negating damage and helps you with the trades, and Chrysalis is really good early with the extra HP that sometimes is the difference between losing or winning the lane.

You can also swap Bone Plating for Revitalize but I still think it's an overkill since Fiora has built in and builds a lot of extra life steal. In hard lanes where you get constantly poked you should definitely swap Chrysalis for Second Wind since it will help you to survive lane until you become strong.

You can also take Resolve first tree with Grasp, Bone Plating, Chrysalis and Revitalize and Precision second with Triumph and Alacrity. If i go Resolve first tree i always go Precision second tree because Fiora feels too clunky without the extra attack speed (for me at least).

When you take Precision first tree you can swap the second tree for Sorcery and take Nimbus Cloak and Transcendence. Nimbus is really good and with that extra move speed it will be much easier to proc the 4 vitals of your ultimate. Transcendence helps Fiora hit that sweet 30% CDR that feels really good on her and helps her a lot. With 30% CDR Q feels like it doesn't have CD and it's really good for escaping and helps for chasing.

The first items are almost always the same. You rush Tiamat into Hydra, unless they buy Thornmail or start stacking armor than go Tiamat into Black Cleaver and finish Hydra after that. This helps her with clearing waves because she has no natural wave clear and is really good about the auto attack resets. After that I like black cleaver, but some times you can take Trinity Force. I like the black cleaver more and build it in 90% of the situations. It gives better stats than Tri Force, is cheaper and Fiora capitalizes better on Black Cleaver. The armor pen. and additional ad is better than the att spd. you get from tri force and sheen is a good item but it isn't a must.

If they have a lot of AP i take Merc's, because you will get that 10% CDR from the Spirit Visage which i take as 3rd item. After that a good item is Maw.
If they are full AD i take Ninja Tabi's and build Guardian Angel 3rd item and Bloodthirster as 4th item or Death's Dance if you need the extra 10% CDR after that.

CDR is so important on Fiora and 30% CDR is almost a must. You can also get 40% CDR but there isn't a lot of difference between 30 and 40 percent on Fiora and I think that 30% is enough. Once you get 30% CDR you get to use Q so often and you are a lot more slippery and sticky. You also have more AA resets with your low CD on E. Also the CDR is sweet for your W.

In the laning phase it's all about wave control. I usually want to push the first wave (but not too hard) and pop vitals whenever i have the chance to. If the vital is placed on the south just move away and you will lose that vital and get a new one. At around level 3-4 I reset the lane. The most important thing about doing the most damage in lane is the AA reset. Make sure that you hit someone when you use your Q because it will have way lower CD than using in a spare space.

General Combo: Proc vital with Q/Auto Attack/AA reset with E/Proc another vital

Also it's very important to properly use W because in some matchups like against Jax or Riven that one skill wins you trades and it's the difference between dying and surviving. At level 6 you can kill most enemies once you get them to 50% of their HP.
After level 6 you will have your Tiamat which also serves as an AA reset, and you can proc vitals even faster. When you are against champions like Garen and Trundle it is very important to know when to pressure and go in and when to back up.

In this phase of the game you should have at least 20% CDR and even maybe 30% CDR if you have built Ionian Boots. I get this boots 75% of the time, because having 30% CDR on Fiora is a must. At this point of the game you should push side lanes and TP if the team needs you. If you don't have TP and your team plans to do an objective you can group with them. Fiora is a pretty good splitpusher because she wins almost every 1v1 at this point and if your team wants to play with splitpusher pretty easily splitpush.
This is the most important rule about team fighting with Fiora. If the enemy ADC isn't next to you DON'T CHASE THEM TO ULT THEM. This is a very common mistake that the players new to Fiora make. You focus the target closest to your team and that dives your team (usually the enemy team jungler or top laner), or the enemy with lowest HP. With your true damage you melt tanks and if you proc 3-4 vitals your team will be healed fully (more than Soraka heal) and that can be game changing. I only use ult on the enemy adcs if they are next to me, pretty much out of position and if I am sure that I will kill them, because if you chase them and they survive you can lose the teamfight.

You should splitpush at this point of the game and make TP plays. You can also make TP flanks and you can melt ADCs with 4 items and your ult, it's almost like a 1 shot. If you can't flank don't chase the enemy adc and focus the tank that dives your team as I said previously. At this point a good W makes the difference between won and lost teamfight. If you get a good W you can stun a valuable target and even if you don't dodging the stun can win you the teamfight.
If you decide to splitpush make sure that everything is warded and if you have 3 items they must send at least 2 people to shut you down because Fiora wins almost every 1v1 late game. If you splitpush you can use your Ultimate for the additional move speed and get away, or if one of the enemies is low you can turn on them and if you kill the one fast you will win the 2v1 and get a double kill.
I like to teamfight more than splitpushing because a good ult heals more than Soraka and will surely win you the fight.